Transcript Document

ERSC 1P92
Assignment 5. Locating plate boundaries on Trafalmador.
The aim of this assignment is to identify plate boundaries and the
location of various geological features on the basis of the spatial
distribution of earthquakes.
Detailed instructions are given in the assignment itself which can be
downloaded from this link.
The following slides were used to give several detailed hints, in class,
as to how to successfully complete the assignment.
Tectonic plate boundaries on Earth are delineated by the distribution of
earthquakes. In this assignment you will look for patterns of
earthquakes on the fictitious planet of Trafalmador in order to identify
plate boundaries and volcanic islands and to predict where mountains
would be found.
Important Points
1. Oceanic trenches
•Deep earthquakes only occur in association with oceanic trenches.
•They are a associated with BOTH shallow and intermediate earthquake.
•Shallow, intermediate and deep earthquakes occur in linear distributions
that are parallel to each other and parallel to the trench.
•The trench is parallel to and ALWAYS on the side of the line or band of
shallow earthquakes where earthquakes are absent.
•This pattern from shallower to deeper earthquakes may extend beneath
continents (a major land mass) or island arc (linear islands that parallel
the trench).
•Earthquake foci become progressively deeper in the direction of
subduction.
2. Oceanic ridges and transform faults
•Oceanic ridges display a linear pattern of earthquakes, all of which
have relatively shallow foci.
•If deep or intermediate earthquakes lie parallel to the band of shallow
earthquakes, it is not a ridge, it is a trench.
•Transform faults displace the oceanic ridge and these faults are also
characterized by a linear pattern of shallow earthquakes.
•Combined the ridge and transform faults form two approximately
perpendicular trending linear patterns of shallow earthquakes.
•Plate movement is away from the ridge axis, in opposite directions on
either side of the ridge.
•If you know the direction of plate movement based on the location of
the trenches and the direction of subduction you can distinguish ridges
from transform faults.
For this assignment you should identify the trenches first.
Deep earthquakes only occur in association with oceanic trenches.
They are a associated with BOTH shallow and intermediate earthquake.
Shallow, intermediate and deep earthquakes occur in linear distributions
that are parallel to each other and parallel to the trench.
The trench is parallel to and ALWAYS on the side of the line or band of
shallow earthquakes where earthquakes are absent.
Earthquake foci become progressively deeper in the direction of
subduction.
Important point: if, along a line that is perpendicular to what you
believe to be there trench, the pattern is not one of shallow, to
intermediate to deep earthquakes along that line (e.g., if it is shallow to
deep or only shallow to intermediate) you are not observing a simple
trench but possibly a more complex situation where two plate boundaries
are in close proximity to each other.
Direction
of
subduction
You can use the direction of subduction to distinguish the subducting
plate from the over-riding plate.
This pattern from shallower to deeper earthquakes away from the trench
may extend beneath continents (a major land mass) or island arc (linear
islands that parallel the trench).
In this example there are long, narrow islands parallel to the trench and
on the over-riding plate. Therefore, these are volcanic islands of an
Island Arc Complex.
When trenches lie parallel to continental masses, the compressive
forces and volcanic activity form linear mountain belts, on the
continent, that parallel the trench like the Andes along the western side
of North America.
Mountain Belt
How steep is
Subduction?
As seen on a map, the
narrower the band of
earthquakes on the map, the
steeper the angle of
subduction.
Important: These cross-sections are comparable
to the cross-sections that you plotted in the
assignment. However, do NOT rely on them to
find location highest and lowest angle of
subduction. Look at the map.
Oceanic ridges are delineated by
a linear pattern of shallow
earthquakes.
If deep or intermediate earthquakes
lie parallel to the band of shallow
earthquake, it is not a ridge, it is a
trench.
Note that Iceland is a
volcanic island that is a part
of the North Atlantic Ridge.
Transform faults displace the oceanic ridge and these faults are also
characterized by a linear pattern of shallow earthquakes.
Combined the ridge and transform faults form two approximately
perpendicular trending linear patterns of shallow earthquakes.
The question is: which of the linear trends of shallow earthquakes is
along the ridge and which is along transform faults?
Plate movement is away from the ridge axis, in opposite directions on
either side of the ridge.
You know the direction of plate movement based on the location of the
trenches so you can determine which linear trending shallow
earthquakes represent the oceanic ridge.
Decide for yourself which of the following figures are correct based on
the direction of plate movement.
OR
Remember:
Transform faults can pass though
continents to connect ridges.
Ridges do not pass through
continents to connect transform
faults.
There are no failed rifts on the
map for assignment 5.